RACECAR/J Build – Chassis

This article marks the start of the prototype RACECAR/J build. RACECAR/J is an 1/10 scale autonomous vehicle. First up, preparing the chassis. Looky here:

Background

This is the third hardware prototype of the RACECAR on JetsonHacks. The prototype base is the MIT RACECAR, an “open-source powerful platform for robotics research and education”.

The platform houses state-of-the-art sensors and computing hardware, placed on top of a powerful 1/10-scale mini race car.

Over time, a couple of different parts have become obsolete since we our first prototype. This includes the Sparkfun IMU, and the TRAXXAS car that we originally built upon. This is not unusual, but still requires some changes to the deck/platform for attachment.

We can break the hardware down into different sections:

Chassis – This version is based on a TRAXXAS Platinum Slash Truck.

Computing – NVIDIA Jetson Development Kit.

Sensors – The robot can use different types of sensors including LIDAR, stereo camera, RGBD cameras, and IMUs.

Electrical – Batteries, Wiring and Interfaces

Mechanical – The “nuts and bolts” that form the backbone of the mechanical structure of the car. This includes the decks.

TRAXXAS Platinum Slash Truck

In this prototype, we build on the TRAXXAS Platinum Slash Truck. In earlier prototypes we used the TRAXXAS Rally which has since been discontinued. However, both the Rally and Slash are similar vehicles built on the same chassis platform. In fact, the Slash meets the demands of our application even better because many of the various suspension bits have been upgraded from plastic to aluminum. This includes the C-hubs, steering blocks, rear hub carriers and axle nuts.

In addition, the Slash does not include a transmitter or receiver. Since these are not used in our project, this provides a little bit of savings.

Chassis Preparation

As shown in the accompanying video, there are several steps in preparing the TRAXXAS Slash. Most of the preparation involves removing parts of the RC Car which which we do not use. Here are the major steps:

Remove the 4 body clips which hold the clear plastic body on the car

Remove the plastic body

Remove the body mounting brackets. There is one in the front, and one in the rear. Each mounting bracket is held in place by two screws.

Remove the stock Electronic Speed Controller (ESC), which is held in place by two screws.

Remove the receiver case. 4 screws hold the cover down, 2 more screws accessible from inside the box hold it to the chassis.

Remove the stock front bumper.

Upgrade the front and rear springs.

Install a new front bumper, the Scalpel Bumper from JConcepts.

Remove the antenna holder

The video gives detailed instructions on the modifications.

Notes

In a previous article, What is the difference between RACECAR project?, we discussed the reasoning behind building our own robot hardware. It’s simple, we want a general purpose self driving platform to better understand the different bits and pieces of autonomous vehicles and the associated software.

There are many ways to modify this build to suite any given application. In this prototype, we replace the stock ESC with a VESC. The VESC is an open source brushless DC motor controller. This provides better control at slow speeds than the stock ESC, as well as the ability to monitor engine speed. The engine speed can be used to calculate crude odometery, since there are no encoders built into the car drivetrain.

The stock plastic body weighs about 6 ounces. The build will be adding 3-5 pounds of batteries, computers and sensors. Therefore a spring upgrade is necessary. The springs shown in the video are the first attempt, but still need to be fine tuned for this application.

Many people have asked for a full bill of material (BOM) for the build. Here’s the deal: Once we’re happy that everything works and fits the bill, we’ll publish the BOM. In addition, we’re setting up a storefront where you can buy the hard to find, custom, and long lead time items. Some of the parts can take up to 10 weeks to get, so we’ll keep some in inventory at the store. We’re still a few weeks from opening the store, but things look quite promising.

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16 Comments

Hi, this is a cool project that I plan on doing as well to better understand the sensors and functions better. Look forward to being able to buying this stuff. Some stuff is just sitting in my amazon shopping cart, but it would be awesome if you added a store as well to find the other parts. Keep up the good work. Really appreciate the documentation.

This is gorgeous! Thank you Tim and JetsonHacks team.
Do you have any update on BOM or store front? I am working on putting together a car and it looks like a bit of work to get all parts ship to Berlin, Germany.
Once again, really appreciate the work 🙂

Hi Shreyas,
We’re working through some issues that have come up over the last couple of months. The first issue is that the external Energizer battery has been discontinued, so we’ve been looking to source an alternative. We’re in the testing stages now. Also, the IMU that was being used in the robot has been superseded. The IMU itself is similar, but has different mechanical mounting requirements which require different layout on the platform deck. We’ll let everyone know the timeline when we are confident everything has come together.

For a battery pack, have you looked into using a USB power bank that supports Qualcomm QuickCharge protocol? QuickCharge 2.0 can deliver 5, 9, 12 or 20V through a USB cable. QuickCharge 3.0 allows finer-grained voltage control. I haven’t found any off-the-shelf controllers to negotiate the voltage, but it looks like a feasible thing to build from a micro controller dev board.

Hi Bob,
As a qualifier, I’m not good at the electrical stuff, so I may be off in the calculations. It doesn’t seem the voltage is an issue, but rather the current.

For the race car, the USB hub needs 2A @ 12 volts, which is about 24 watts. For the Jetson, the claim for the module alone is 15W @ 19V (0.8A), which is around 1.2A @ 12 volts. Once you add in the carrier board, it’s probably closer to twice that, around 2.5A @ 12V.

The Energizer 18000 Spec:
Rated Output Current
DC5V, 2100 mA
DC12V, 2000 mA
DC19V, 3500 mA
It’s a beast for this application, and weighs 1.14 lbs. In the original MIT application, the 12V output drives the USB hub, the 19V drives the Jetson and the Hokuyo LIDAR.

Notice two things. First the number of Amps available at higher voltages. Second, there are two 12+V outputs. Few power banks that I’ve been able to find have two 12+ volt outputs. Some have a single 12V along with their 5V USB.

The underlying issue is that most power bank configurations (5V, 12V) don’t provide enough amperage on the given ports. Most 5V ports max out around 2.5A. For a dual voltage power bank with a 2A@12V output, let’s say the 5V USB ports need to provide another 2A@12V. Most 5V USB outputs on power banks are < 2.5A. If a voltage converter is used, 2.5A@5V gives around 1A@12V give or take, so you'd need a couple of voltage converters. You could probably get it to work with additional circuitry, but it's more challenging. On the 12V side, the power banks tend to max out around 3A@12V, which is *almost* enough, but this isn't horse shoes or hand grenades.
If I was the only one using it, I would probably just go LiPo (2S or 3S) with a voltage converter and a power distribution block. The same could be done with safer battery chemistries, like NiMh or LiFePO4. But what I noticed in tests with different groups of high school students, the expectation is that the batteries are like that of a smart phone with all the battery management systems in place. The students don't worry about over charging or discharging the battery, or that the battery would actually catch fire if treated poorly. That's the advantage of having the LiPo battery inside of a power bank, all that circuitry is built-in.
Nirvana is to run everything off one battery pack, but sharing electronics with the car motor is a little challenging because the motor both dirties the power supply and can cause brownouts under heavy load. With additional circuitry, this can be addressed. However, I'm not an electrical engineer so that's a bit of a challenge.
With that said, we've found a couple of candidates which are now going through testing. I'm not thrilled with them as each have tradeoffs, but they appear to work.

There are a couple of issues you should be aware of with that selection:
1) There is an output for 20V, the maximum for the Jetson is 19.5V as I recall. You would have to consider using some type of voltage converter (a buck converter probably) so that you don’t damage the Jetson.
2) The battery is heavy. It’s about 2.5x heavier than the Energizer. You should take that in to consideration when configuring the chassis.

Ah thanks! Appreciate your feedback 🙂 You mentioned in above comments that you are testing couple of candidates. Could you please list them here? I am in US till next weekend and maybe I can already buy them 🙂

Still on, it’s just taken a little bit longer than expected due to some changes in the parts being used. The store sells the latest bits and pieces (except for the cameras and lidars) which takes most of the pain out of the build. Unless you have a laser cutter or CNC machine, for example, getting the platforms made is a little frustrating. Over the next couple of weeks we’ll cover building a RACECAR, installing the software stack, and so on. Still, we hope people experiment. That’s why we came up with the ‘FlatNose’ platform deck with mounting holes for a RPLIDAR A2, for example. Or leave the lidars off all together and use just cameras, or add ultra-sonics, or hopefully stuff we haven’t even thought of yet!